According to a halftoning process used in color electrophotography, it is customary to form a halftone threshold matrix (referred to simply as a threshold matrix) with a rational screen having a small halftone cell size (see Japanese Laid-Open Patent Publication No. 2002-112047, paragraph [0015]).
In the art of AM screens, assuming there are 8×8 or 16×16 pixels that can be blackened (colored), then the halftone cell size refers to the size 8×8 or 16×16. Assuming the screen angle is 0°, then the size of a threshold matrix, made up of thresholds corresponding to respective pixels, is equal to the halftone cell size. In the halftone cell, blackened (colored) areas are referred to as halftone dots.
In rational screens, (sizes of) halftone cells are identical to each other. When halftone dots are generated by comparing a continuous tone image with the threshold matrix, then the halftone dots of each halftone cell have identical sizes and shapes. In the present specification, the rational screen refers to a screen wherein, if a screen angle θ is expressed as θ=arctan(m/n) using a trigonometric arctangent where m and n are natural numbers, then the size of the threshold matrix satisfies a positive integral multiple of m2+n2.
However, since the halftone cell size (and the halftone matrix size) of rational screens is small, a rosetta pattern optimization, which has been taken into account for offset printing, cannot be performed. That is, the C, M, K plates cannot be set accurately to angles at 30° intervals, e.g., 15°, 45°, 75°, and hence moiré patterns are generated.
In order to angularly space the C, M, K plates exactly at 30° intervals, it could be possible to generate halftone dots with a supercell having a large threshold matrix size, rather than using a rational screen.